Hot melt magnetic sealant tapes are widely used in the automotive industry. The tapes are applied to seal the gaps between the adjoining ferrometallic components of automobile bodies and to thus, for example, prevent the entry of water and exhaust gases into the passenger compartment of the vehicle.
A joint to be sealed is covered with a hot melt magnetic sealant tape. The magnetic properties of the sealant hold the tape in position temporarily. The tape is subsequently heated, typically at the time when the automobile body undergoes an elevated temperature paint bake cycle. As the temperature of the tape is increased, the viscosity of the tape decreases and the softened tape flows and is drawn into conformance with the contour of the joint by gravity and by magnetic forces to, ideally, produce a void-free seal.
While the magnetic properties of the tape serve to temporarily position the tape and to draw the tape into contact with the substrate, the long term performance of the seal depends upon the development of chemical adhesion between the tape and the substrate.
A large number of commercially important automotive applications for hot melt magnetic sealant tapes require adhesion to steel substrates that have been protected with an epoxy based electrodeposition coating (e.g. Uniprime.RTM. coatings available from Pittsburg Plate Glass, PPG). Conventional sealant tape formulations develop acceptable adhesion to such epoxy coatings only after an elevated temperature bake of about 30 minutes duration at temperatures of 325.degree. F. or above. The requirements of such an elevated temperature bake have typically been satisfied during automotive paint baking cycles and conventional magnetic tape sealants have been widely used in the automotive industry to seal joints between epoxy electrodeposition coated substrates. However, a recent shift toward lower temperature (between about 240.degree. F. and about 300.degree. F.) paint baking processes has rendered the conventional magnetic sealant tapes much less attractive in many applications.
A smaller, yet still significant, number of commercially important automotive applications for hot melt magnetic sealant tapes require adhesion to galvanized steel substrates that are coated with a film of conventional metal working oil. Conventional sealant tapes do not offer acceptable adhesion to the oil coated galvanized steel substrates, regardless of the severity of the processing conditions.
What is needed in this art is a hot melt magnetic sealant tape that overcomes some of the above difficulties.